1. Field of the Invention
The present invention relates to a hard magnetic alloy having excellent magnetic characteristics and temperature-dependent properties and used in sensors such as magnetic encoders and potentiometers, motors, actuators, and speakers. The present invention relates to a hard magnetic alloy compact and a method for producing the alloy and the compact.
2. Description of the Related Art
Ndxe2x80x94Fexe2x80x94B magnets and Smxe2x80x94Co magnets are generally known as magnetic materials which show superior characteristics to that of ferrite magnets and Alnico (Alxe2x80x94Nixe2x80x94Coxe2x80x94Fe) magnets. Novel alloy magnets having further improved characteristics and particularly Smxe2x80x94Fexe2x80x94N magnets have also been intensively studied. These magnets, however, must contain at least 10 atomic % of Nd or at least 8 atomic % of Sm. Use of large quantities of expensive rare earth elements inevitably increases the production costs. Since the magnetic characteristics of Ndxe2x80x94Fexe2x80x94B magnets are largely dependent on temperature, they cannot be used as sensors. On the other hand, Smxe2x80x94Co magnets have not been used in practice in spite of their smaller thermal coefficients of magnetization, because they are more expensive than the Ndxe2x80x94Fexe2x80x94B magnets.
Ferrite magnets and Alnico magnets are inexpensive compared to the rare earth magnets; however, the ferrite magnets have larger thermal coefficients of magnetization and thus cannot be used as sensors, whereas the Alnico magnets have extremely low coercive forces.
The above-mentioned hard magnetic alloys have been produced by spraying molten alloys onto rotating drums to form thin ribbons by quenching the alloys or by spraying molten alloys into cooling gas to form alloy powders by quenching alloy droplets. The thin ribbons and alloy powders must therefore be formed into given shapes before being used for motors, actuators, and speakers.
Typical methods for molding magnetic powder include compaction molding and injection compacting of a mixture of the magnetic powder and a rubber or plastic binder. The resulting magnet is referred to as a xe2x80x9cbond magnetxe2x80x9d. Since the versatility of possible form features of bond magnets is high, they have been widely used in electronic parts. The binder in bond magnets, however, causes inferior magnetic characteristics because of decreased remanent magnetization and low mechanical strength of the bond magnet
Accordingly, the advent of inexpensive magnetic materials having hard magnetic characteristics superior to those of ferrite magnets and excellent temperature-dependent properties has been eagerly awaited
The present inventors have studied inexpensive hard magnetic materials having excellent magnetic characteristics and temperature-dependent properties, and have discovered from various experimental results that the thermal coefficient of magnetization is related to the permeance factor p.
Also, the present inventors have directed their attention to the heating rate during annealing of a quenched alloy essentially consisting of an amorphous phase and discovered that hard magnetic characteristics are related to the nano-crystalline structure (particularly, crystal grain size of the bcc(body centered cubic)-Fe phase) in a fine crystalline phase which is precipitated by the annealing.
It is an object of the present invention to provide a hard magnetic alloy which is capable of low cost production and has excellent hard magnetic characteristics and excellent temperature-dependent properties.
It is another object of the present invention to provide a hard magnetic alloy compact having high mechanical strength and excellent magnetic characteristics.
It is a further object of the present invention to provide a method for producing the hard magnetic alloy or hard magnetic alloy compact.
A first aspect of the present invention is a hard magnetic alloy comprising at least one element T selected from the group consisting of Fe, Co and Ni, at least one rare earth element R, and B, the hard magnetic alloy containing at least 10 percent by volume of a soft magnetic or semi-hard magnetic phase having a coercive force of 1 kOe or less and at least 10 percent by volume of a hard magnetic phase having a coercive force of 1 kOe or more, the absolute value of the thermal coefficient of magnetization being 0.15%/xc2x0 C. or less when the hard magnetic alloy is used in a shape causing a permeance factor of 2 or more.
Preferably, the hard magnetic alloy may primarily contain a fine crystalline phase having an average crystal grain size of 100 nm or less.
Preferably, the absolute value of the thermal coefficient of magnetization may be 0.1%/xc2x0 C. or less when the hard magnetic alloy is used in a shape causing a permeance factor of 10 or more.
Preferably, the ratio Ir/Is of the remanent magnetization Ir to the saturation magnetization Is may be 0.6 or more.
Preferably, the hard magnetic alloy may have the following formula:
TxMyRzBw
wherein T represents at least one element selected from the group consisting of Fe, Co and Ni, M represents at least one element selected from the group consisting of Zr, Nb, Ta and Hf, R represents at least one rare earth element, and the suffixes x, y, z and w by atomic percent satisfy 50xe2x89xa6x, 0xe2x89xa6yxe2x89xa615, 3xe2x89xa6zxe2x89xa620, and 2xe2x89xa6wxe2x89xa620, respectively. Preferably, the suffixes x, y, z and w by atomic percent may satisfy 80xe2x89xa6xxe2x89xa692, 1xe2x89xa6yxe2x89xa65, 3xe2x89xa6zxe2x89xa610, and 3xe2x89xa6wxe2x89xa67, respectively.
Preferably, the hard magnetic alloy may have the following formula:
TxMyRzBwSiu
wherein T represents at least one element selected from the group consisting of Fe, Co and Ni, M represents at least one element selected from the group consisting of Zr, Nb, Ta and Hf, R represents at least one rare earth element, and the suffixes x, y, z, w, and u by atomic percent satisfy 50xe2x89xa6x, 0xe2x89xa6yxe2x89xa615, 3xe2x89xa6zxe2x89xa620, 2xe2x89xa6wxe2x89xa620, and 0xe2x89xa6uxe2x89xa65, respectively. Preferably, the suffixes x, y, z, w, and u by atomic percent may satisfy 80xe2x89xa6xxe2x89xa692, 1xe2x89xa6yxe2x89xa65, 3xe2x89xa6zxe2x89xa610, 3xe2x89xa6wxe2x89xa67, and 0.5xe2x89xa6uxe2x89xa65, respectively.
Preferably, the hard magnetic alloy may have the following formula:
TxMyRzBwEv
wherein T represents at least one element selected from the group consisting of Fe, Co and Ni, M represents at least one element selected from the group consisting of Zr, Nb, Ta and Hf, R represents at least one rare earth element, E represents at least one element selected from the group consisting of Cr, Al, Pt and platinum elements, and the suffixes x, y, z, w, and v by atomic percent satisfy 50xe2x89xa6x, 0xe2x89xa6yxe2x89xa615, 3xe2x89xa6zxe2x89xa620, 2xe2x89xa6wxe2x89xa620, and 0xe2x89xa6vxe2x89xa610, respectively. Preferably, the suffixes x, y, z, w, and v by atomic percent may satisfy 80xe2x89xa6xxe2x89xa692, 1xe2x89xa6yxe2x89xa65, 3xe2x89xa6zxe2x89xa610, 3xe2x89xa6wxe2x89xa67, and 0xe2x89xa6vxe2x89xa65, respectively.
Preferably, the hard magnetic alloy may have the following formula:
TxMyRzBwEvSiu
wherein T represents at least one element selected from the group consisting of Fe, Co and Ni, M represents at least one element selected from the group consisting of Zr, Nb, Ta and Hf, R represents at least one rare earth element, E represents at least one element selected from the group consisting of Cr, Al, Pt and platinum elements, and the suffixes x, y, z, w, v, and u by atomic percent satisfy 50xe2x89xa6x, 0xe2x89xa6yxe2x89xa615, 3xe2x89xa6zxe2x89xa620, 2xe2x89xa6wxe2x89xa620, 0xe2x89xa6vxe2x89xa610, and 0xe2x89xa6uxe2x89xa65, respectively. Preferably, the suffixes x, y, z, w, v, and u by atomic percent satisfy 80xe2x89xa6xxe2x89xa692, 1xe2x89xa6yxe2x89xa65, 3xe2x89xa6zxe2x89xa610, 3xe2x89xa6wxe2x89xa67, 0xe2x89xa6vxe2x89xa65, and 0.5xe2x89xa6uxe2x89xa65, respectively.
A second aspect of the present invention is a method for producing a hard magnetic alloy comprising the steps of: preparing an alloy containing at least one element T selected from the group consisting of Fe, Co and Ni, at least one rare earth element R, and B, and essentially consisting of an amorphous phase by a liquid quenching process, and annealing the alloy at a heating rate of 10xc2x0 C./min. or more.
Preferably, a fine crystalline phase having an average crystal grain size of 100 nm or less may be precipitated as a main phase by the annealing.
Preferably, the hard magnetic alloy in this method may have one of the above-mentioned composition.
A third aspect of the present invention is a hard magnetic alloy compact comprising an Fe-based or FeCo-based alloy containing 3 to 20 atomic percent of at least one rare earth element R, and 2 to 20 atomic percent of B, wherein the alloy having a texture, in which at least a part or all of the texture comprises an amorphous phase or fine crystalline phase having an average crystal grain size of 100 nm or less, is subjected to crystallization or grain growth under stress, such that a mixed phase composed of a soft magnetic or semi-hard magnetic phase and a hard magnetic phase is formed in the texture, anisotropy is imparted to the crystal axis of the hard magnetic phase, and the hard magnetic alloy compact has a coercive force of 1 kOe or more.
Preferably, the hard magnetic alloy compact may comprise at least 10 percent by volume of a soft magnetic or semi-hard magnetic phase having a coercive force of 1 kOe or less which comprises a body centered cubic (bcc) Fe phase or bcc-FeCo phase, an Fexe2x80x94B compound phase, and an amorphous phase as precipitates, and at least 10 percent by volume of a hard magnetic phase having a coercive force of 1 kOe or more which comprises an R2Fe14B phase, wherein R represents at least one rare earth element.
Preferably, the annealed alloy may be crystallized or may be subjected to crystal growth under stress and may be simultaneously compacted.
Preferably, the hard magnetic alloy contains an amorphous phase, and may be formed by solidifying an alloy having hard magnetic characteristics when being crystallized, by means of a softening phenomenon of the alloy during the crystallization reaction.
Preferably, the alloy may be heated under stress.
Preferably,the relative density of the compact obtained by compacting the alloy is 90% or more.
Preferably, the hard magnetic alloy compact may have a remanent magnetization of 100 emu/g or more.
Preferably, the ratio of the remanent magnetization Ir to the saturation magnetization Is may be 0.6 or more Preferably, the hard magnetic alloy compact has one of the composition described in the first aspect.
A fourth aspect of the present invention is a method for producing a hard magnetic alloy compact comprising the following steps of: quenching an Fe- or FeCo-based alloy containing 3 to 20 atomic percent of at least one rare earth element R and 2 to 20 atomic percent of B so as to form a texture, in which at least a part or all of the texture comprises an amorphous phase or fine crystalline phase having an average crystal grain size of 100 nm or less; performing crystallization or grain growth of the alloy under stress, such that a mixed phase composed of a soft magnetic or semi-hard magnetic phase and a hard magnetic phase is formed in the texture; imparting anisotropy to the crystal axis of the hard magnetic phase.
Preferably, after performing crystallization or grain growth of the alloy under stress, the alloy may be annealed at 400xc2x0 C. to 1,000xc2x0 C. so as to precipitate a fine crystalline phase having an average crystal grain size of 100 nm or less as a main phase in the texture.
Preferably, the alloy after quenching may be compacted while performing crystallization or grain growth of the alloy under stress.
Preferably, the hard magnetic alloy contains an amorphous phase, and may be formed by solidifying an alloy having hard magnetic characteristics when being crystallized, by means of a softening phenomenon of the alloy during the crystallization reaction.
Preferably the alloy may be heated under stress.